Update .travis.yml to unblock Ruby 2.2 builds (#628)

* Update Ruby 2.2.7 to 2.2.10

There are 3 patch versions of security and vulnerability fixes and 2.2.7
was released back in March 28 2017.

* Use older version of RubyGems for Ruby 2.2

The current build tries to update to RubyGems 3. This currently fails
for Ruby 2.2 because RubyGems 3.0.0 drops support for Ruby 2.2.X.

This change should support Ruby 2.2 by explicitly installing RubyGems
2.7 after failing to find a compatible version of 3.

Author: drwl

.travis.yml

@@ -1,7 +1,7 @@
 sudo: false
 language: ruby
 rvm:
-- 2.2.7
+- 2.2.10
 - 2.3.8
 - 2.4.6
 - 2.5.5
@@ -16,7 +16,7 @@ matrix:
   - rvm: ruby-head
 
 before_install:
-- gem update --system
+- gem update --system || (gem i "rubygems-update:~>2.7" --no-document && update_rubygems)
 - gem update bundler
 
 script: