remove unneeded travis configurations

Author: saghm

.travis.yml

@@ -13,41 +13,16 @@ before_install:
   - gem update --system
   - gem update bundler
 
-matrix:
-  fast_finish: true
-  exclude:
-    - rvm: ruby-head
-      gemfile: gemfiles/rails_master.gemfile
-    - rvm: jruby-9.1.15.0
-      gemfile: gemfiles/rails_master.gemfile
-    - rvm: jruby-9.1.15.0
-      gemfile: gemfiles/driver_master.gemfile
-    - rvm: jruby-9.1.15.0
-      env: MONGODB=2.6.12
-    - rvm: jruby-9.1.15.0
-      env: MONGODB=3.4.10
-  allow_failures:
-    - rvm: ruby-head
-    - gemfile: gemfiles/rails_master.gemfile
-
 rvm:
-  - 2.5
-  - ruby-head
-  - jruby-9.1.15.0
+  - 2.4.1
 
 gemfile:
   - Gemfile
-  - gemfiles/driver_master.gemfile
-  - gemfiles/rails_master.gemfile
 
 env:
   global:
     - CI="travis"
-    - JRUBY_OPTS="--server -J-Xms512m -J-Xmx1G"
-  matrix:
-    - MONGODB=2.6.12
-    - MONGODB=3.4.10
-    - MONGODB=3.6.2
+    - MONGODB=3.4.17
 
 before_script:
   - wget http://fastdl.mongodb.org/linux/mongodb-linux-x86_64-${MONGODB}.tgz -O /tmp/mongodb.tgz