Add Tarantool v2.11 to CI, remove older revions

Author: viciious

.travis.yml

@@ -32,11 +32,9 @@ install:
 matrix:
   include:
     - env: TARANTOOL_VER=1.6
-    - env: TARANTOOL_VER=1.7
     - env: TARANTOOL_VER=1.10
-    - env: TARANTOOL_VER=2.3
-    - env: TARANTOOL_VER=2.5
     - env: TARANTOOL_VER=2.6
+    - env: TARANTOOL_VER=2.11
 
 after_failure:
 - cat /home/travis/tntlog/*