remove 1.8 and ruby-head from travis

.travis.yml

@@ -3,10 +3,8 @@ services: mongodb
 script: "bundle exec rake spec"
 
 rvm:
-  - 1.8.7
   - 1.9.3
   - 2.0.0
-  - ruby-head
 
 gemfile:
   - gemfiles/active_record_32.gemfile
@@ -14,8 +12,3 @@ gemfile:
   - gemfiles/mongo_mapper.gemfile
   - gemfiles/mongoid_30.gemfile
   - gemfiles/sinatra.gemfile
-
-matrix:
-  exclude:
-    - rvm: 1.8.7
-      gemfile: gemfiles/mongoid_30.gemfile