Travis: gem install bundler

[olleolleolle]

This PR adds a before_install step to install a Bundler, in order to see if jruby-head can build.

Update: That did it. It now runs, see Travis build configured on my fork.

And, comes up with a test failure on jruby-head:

  1) Failure:
Supported types#test_0010_should support generic time type [/home/travis/build/olleolleolle/sequel/spec/integration/type_test.rb:83]:
Expected: "'19:49:16.047188'"
  Actual: "'19:49:16.047000'"

[jeremyevans]

This looks fine to me, but can you please rebase this into a single commit so there isn't so much code churn?

[olleolleolle]

@jeremyevans There.

[janko]

Awesome, I'm having the same problem in my open source project, it's great to know there is a fix!