Fix release and PGP maintainer links in README and CONTRIBUTING (#355)

* In the introductory section of README.md, the pre-built binaries link
was pointing to version `v0.4.0` and the current version is `0.7.0`.

It would be better to have a dynamic pointing to releases using
github's `/releases/latest` endpoint.

* The maintainer PGP key link pointee to a invalid link ( blog dot dlsouza
 dot lol at assets/pgp.asc give a 404 HTTP error).

This commit simple changes `pgp.asc` to `gpg.asc` in the refered link
in both README.md and CONTRIBUTING.md.

CONTRIBUTING.md

@@ -189,7 +189,7 @@ pub fn validate_block_time(
 Security
 --------
 
-Given the critical nature of Floresta as a node implementation, we take security very serious. If you have any security vulnerability to report, please send it to `me AT dlsouza DOT lol` preferentially using my [PGP key `2C8E0F 836FD7D BBBB9E 9B2EF899 64EC3AB 22B2E3`](https://blog.dlsouza.lol/assets/pgp.asc).
+Given the critical nature of Floresta as a node implementation, we take security very serious. If you have any security vulnerability to report, please send it to `me AT dlsouza DOT lol` preferentially using my [PGP key `2C8E0F 836FD7D BBBB9E 9B2EF899 64EC3AB 22B2E3`](https://blog.dlsouza.lol/assets/gpg.asc).
 
 Testing
 -------

README.md

@@ -20,7 +20,7 @@
 Welcome to Floresta, a lightweight Bitcoin full node implementation written in Rust, powered by [Utreexo](https://eprint.iacr.org/2019/611) a novel dynamic accumulator  designed for the Bitcoin UTXO set.
 
 This project is composed of two parts, `libfloresta` and `florestad`. `libfloresta` is
-a set of reusable components that can be used to build Bitcoin applications. `florestad` is built on top of `libfloresta` to provide a full node implementation, including a watch-only wallet and an Electrum server. If you just want to run a full node, you can use `florestad` directly, either by building it from source or by downloading a pre-built binary from the [releases](https://github.com/vinteumorg/Floresta/releases/tag/v0.4.0).
+a set of reusable components that can be used to build Bitcoin applications. `florestad` is built on top of `libfloresta` to provide a full node implementation, including a watch-only wallet and an Electrum server. If you just want to run a full node, you can use `florestad` directly, either by building it from source or by downloading a pre-built binary from the [releases](https://github.com/vinteumorg/Floresta/releases/latest).
 
 If you want to use `libfloresta` to build your own Bitcoin application, you can find the documentation [here](https://docs.getfloresta.sh/floresta/).
 
@@ -49,7 +49,7 @@ If you want to use `libfloresta` to build your own Bitcoin application, you can
 ## Community
 
 If you want to discuss this project, you can join our Discord server [here](https://discord.gg/5Wj8fjjS93). If you want to disclose
-a security vulnerability, please email `Davidson Souza at me AT dlsouza DOT lol`, using the PGP key [`2C8E0F 836FD7D BBBB9E 9B2EF899 64EC3AB 22B2E3`](https://blog.dlsouza.lol/assets/pgp.asc).
+a security vulnerability, please email `Davidson Souza at me AT dlsouza DOT lol`, using the PGP key [`2C8E0F 836FD7D BBBB9E 9B2EF899 64EC3AB 22B2E3`](https://blog.dlsouza.lol/assets/gpg.asc).
 
 ## Building