Add staticcheck to .travis.yml
Add staticcheck to .travis.yml

Add the $PKGS var, to avoid having to type $(go list ./... | grep -v
/vendor/) over and over.
Add leading and trailing slashes to vendor (vendor -> /vendor/) so that
a go files containing "vendor" won't be filtered.
Remove unnecessary indentation.
Add the before_script section, so $PKGS can be calculated and
staticcheck can be fetched without polluting the logs.
Add the $STATICCHECK_IGNORE var, which tells staticcheck to ignore the
deprecation warning for filepath.HasPrefix in two files.

golang/dep,EwanValentine/dep,ibrasho-forks/dep,EwanValentine/dep,ibrasho-forks/dep,golang/dep,golang/dep