Add gflags dependency to Travis script

Summary:
Travis is failing due to missing gflags
(https://travis-ci.org/facebook/rocksdb/jobs/80307921). Adding libgflags-dev
should help.

Test Plan: Run tests on Travis.

Reviewers: rven, anthony, yhchiang, aekmekji, igor, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D46905

.travis.yml

@@ -9,7 +9,7 @@ matrix:
       addons:
          apt:
             sources: ['ubuntu-toolchain-r-test', 'llvm-toolchain-precise-3.6']
-            packages: ['clang-3.6', 'zlib1g-dev', 'libbz2-dev', 'libsnappy-dev']
+            packages: ['clang-3.6', 'zlib1g-dev', 'libbz2-dev', 'libsnappy-dev', 'libgflags-dev']
     - os: osx
       compiler: clang
 

build_tools/build_detect_platform

@@ -213,7 +213,7 @@ EOF
     fi
 
     # Test whether gflags library is installed
-    # http://code.google.com/p/gflags/
+    # http://gflags.github.io/gflags/
     # check if the namespace is gflags
     $CXX $CFLAGS -x c++ - -o /dev/null 2>/dev/null << EOF
       #include <gflags/gflags.h>