Don't warn about cl-functions with old cl-lib

* .travis.yml (install): Set warnings to (not cl-functions) if cl-lib
needs to be downloaded.  Since the downloaded cl-lib just aliases cl.el
functions, trying to use cl- prefixed functions would cause compile
warnings otherwise.
* Rakefile: Pass environment variable `warnings' as
`byte-compile-warnings' value when compiling.

Rakefile

@@ -99,6 +99,7 @@ desc "Compile yasnippet.el into yasnippet.elc"
 
 rule '.elc' => '.el' do |t|
   sh "#{$EMACS} --batch -L . --eval \"(setq byte-compile-error-on-warn t)\"" +
+     " --eval \"(setq byte-compile-warnings #{ENV['warnings']})\"" +
      " -f batch-byte-compile #{t.source}"
 end
 task :compile => FileList["yasnippet.el"].ext('elc')