Clarify OSX build does not require Ninja yet.

This is a follow up CL for 276fa5c,
which mistakenly stated that OSX build requires Ninja.

Although we still plan to use Ninja for OSX build as mentioned
in #247, it is not yet done.  Until then, we don't have to ask people
to install Ninja when building Mozc for OSX.

BUG=#247
TEST=unittest
REF_BUG=
REF_CL=

.travis.yml

@@ -32,10 +32,6 @@ os:
 
 language: objective-c
 
-install:
-  - git -C src/third_party clone https://chromium.googlesource.com/chromium/tools/depot_tools.git
-  - export PATH="$PATH":`pwd`/src/third_party/depot_tools
-
 script:
   - cd ./src
   - GYP_DEFINES="mac_sdk=10.9 mac_deployment_target=10.8" python build_mozc.py gyp --noqt

doc/build_mozc_in_osx.md

@@ -10,7 +10,6 @@ We only support OS X 10.7 or later intel only.
 Building on Mac requires the following software.
 
   * Xcode
-  * [Ninja](https://github.com/martine/ninja)
 
 If you don't need to run gui tools like about dialog, config dialog, or dictionary tool, you can omit installing Qt.  Candidate window still shows without Qt.  See below for the detailed information.