Delete Gemfile.lock

Avoid [failures in travis](https://travis-ci.org/google/protobuf/jobs/163611664) under jruby

ruby/Gemfile.lock

-PATH
-  remote: .
-  specs:
-    google-protobuf (3.0.0.alpha.5.0.5)
-
-GEM
-  remote: https://rubygems.org/
-  specs:
-    power_assert (0.2.2)
-    rake (10.4.2)
-    rake-compiler (0.9.5)
-      rake
-    rake-compiler-dock (0.5.1)
-    rubygems-tasks (0.2.4)
-    test-unit (3.0.9)
-      power_assert
-
-PLATFORMS
-  java
-  ruby
-
-DEPENDENCIES
-  google-protobuf!
-  rake-compiler
-  rake-compiler-dock
-  rubygems-tasks
-  test-unit
-
-BUNDLED WITH
-   1.11.2