Add rails 5.1.0 to the travis matrix (rspec#1809)

Author: Sam Phippen

.travis.yml

@@ -61,19 +61,19 @@ matrix:
     - rvm: 2.2.2
       env: RAILS_VERSION=5-0-stable
     - rvm: 2.2.2
-      env: RAILS_VERSION=5.1.0.rc2
+      env: RAILS_VERSION=5.1.0
     - rvm: 2.3.3
       env: RAILS_VERSION=master
     - rvm: 2.3.3
       env: RAILS_VERSION=5-0-stable
     - rvm: 2.3.3
-      env: RAILS_VERSION=5.1.0.rc2
+      env: RAILS_VERSION=5.1.0
     - rvm: 2.4.0
       env: RAILS_VERSION=master
     - rvm: 2.4.0
       env: RAILS_VERSION=5-0-stable
     - rvm: 2.4.0
-      env: RAILS_VERSION=5.1.0.rc2
+      env: RAILS_VERSION=5.1.0
   exclude:
     # 3.0.x is not supported on MRI 2.0+
     - rvm: 2.0.0