No need to update rubygems on Travis CI any more.

This `before_install` block was added in [1], because of a bug in the version
of Rubygems provided by Travis CI by default (v2.2.0). We need a fix which is in
Rubygems v2.2.1 and the default version provided by Travis CI is now v2.2.2.

[1] 3128fb8

.travis.yml

@@ -1,6 +1,3 @@
-before_install:
-- gem update --system
-- gem --version
 script: "rake test"
 rvm:
   - 1.8.7