TravisCI: Remove a couple of linters.

The linters take quite a while to run in the TravisCI environment which
causes the deadline to be exceeded.

While here, also increase the allowed deadline for the remaining linters
to 10 minutes and also update Travis to use a new ubuntu distribution.

.travis.yml

@@ -1,3 +1,4 @@
+dist: trusty
 language: go
 go:
   - 1.7.x

goclean.sh

@@ -2,10 +2,8 @@
 # The script does automatic checking on a Go package and its sub-packages, including:
 # 1. gofmt         (http://golang.org/cmd/gofmt/)
 # 2. go vet        (http://golang.org/cmd/vet)
-# 3. gosimple      (https://github.com/dominikh/go-simple)
-# 4. unconvert     (https://github.com/mdempsky/unconvert)
-# 5. race detector (http://blog.golang.org/race-detector)
-# 6. test coverage (http://blog.golang.org/cover)
+# 3. race detector (http://blog.golang.org/race-detector)
+# 4. test coverage (http://blog.golang.org/cover)
 
 # gometalinter (github.com/alecthomas/gometalinter) is used to run each each
 # static checker.
@@ -31,8 +29,6 @@ linter_targets=$(glide novendor)
 test -z "$(gometalinter --disable-all \
 --enable=gofmt \
 --enable=vet \
---enable=gosimple \
---enable=unconvert \
---deadline=4m $linter_targets 2>&1 | tee /dev/stderr)"
+--deadline=10m $linter_targets 2>&1 | tee /dev/stderr)"
 
 env GORACE="halt_on_error=1" go test -short -race $linter_targets