Revert back the condition as the issue is with Travis CI OSX build env.

[ci skip]

A new issue is opened at their repo: https://github.com/travis-ci/travis-ci/issues/2085.
If it could not be resolved quickly then we will force to export that environment variable in our own .travis.yml file.

Source/CMake/Modules/Urho3D-CMake-common.cmake

@@ -216,7 +216,7 @@ else ()
         endif ()
     elseif (IOS)
         # When performing CI build, suppress all the warnings for iOS build because 3rd party libraries produce too many of them and yet we don't want to touch the 3rd party library's source codes unnecessarily
-        if ("$ENV{CI}" STREQUAL true)
+        if ($ENV{CI})
             set (CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -w")
             set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -w")
         endif ()