Dont 'include -stdlib=libc++ in CXXFLAGS in Travis build script

There is no need to specify it explicitly any more and it seems to confuse
make because of an apparent problem with (insufficient?) quoting in Mac
builds.

.travis.yml

@@ -21,7 +21,7 @@ matrix:
         - os: osx
           osx_image: xcode6.4
           compiler: clang
-          env: wxCONFIGURE_FLAGS="--enable-cxx11" wxMAKEFILE_FLAGS="CXXFLAGS='-std=c++11 -stdlib=libc++'" wxSKIP_SAMPLES=1
+          env: wxCONFIGURE_FLAGS="--enable-cxx11" wxMAKEFILE_FLAGS="CXXFLAGS=-std=c++11" wxSKIP_SAMPLES=1
 
 branches:
     only: