Re-enable linux on travis

Summary: Travis clang issue on linux is fixed now, re-enable linux on travis

Test Plan: https://travis-ci.org/facebook/rocksdb/builds/141098948

Reviewers: yiwu, sdong, yhchiang, andrewkr

Reviewed By: andrewkr

Subscribers: andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D60189

.travis.yml

@@ -1,7 +1,7 @@
 sudo: false
 language: cpp
 os:
-#  - linux
+  - linux
   - osx
 compiler:
   - clang