Remove deprecated Ruby version from Travis CI

Recent versions of Rake do not support Ruby 1.8.7.
As of early 2018, Ruby 2.2 is the oldest supported version.
Versions 2.2.7, 2.3.4 & 2.4.1 are the ones preinstalled in Travis.

Author: Sigill

.travis.yml

@@ -1,9 +1,8 @@
 language: ruby
 rvm:
-  - 1.8.7
-  - 2.0.0
-  - 2.1.1
-  - 2.3.0
+  - 2.2.7
+  - 2.3.4
+  - 2.4.1
 addons:
   apt:
     packages: