[Android] remove NDK directory before installing new one
[Android] remove NDK directory before installing new one

If "~/.rubymotion-android/ndk" was already existed,
"~/.rubymotion-android/ndk" will not be updated as expected.

```
FileUtils.mv(File.join(@tmp_directory, "android-ndk-#{@ndk_version}"), @ndk_directory)
```
line just move the android-ndk-e10d directory in @ndk_directory,
like "~/.rubymotion-android/ndk/android-ndk-e10d"

katsuyoshi/RubyMotion,katsuyoshi/RubyMotion,hboon/RubyMotion,HipByte/RubyMotion,katsuyoshi/RubyMotion,HipByte/RubyMotion,hboon/RubyMotion,HipByte/RubyMotion,katsuyoshi/RubyMotion,hboon/RubyMotion