Fix naming of emulator images for new targetAbis

Hi,

It is currently not possible to create Android Wear, Android TV, nor Android 5.0 with Google APIs emulator images with the plugin.

Previously, Tag/ABIs were either
* armeabi
* armeabi-v7a
* x86
* x86_64
* mips

Google seems to have adopted a new convention on targets, now it can also be (for example) :
* android-wear/armeabi-v7a (for Android Wear)
* google_apis/x86_64 (For 5.0 with Google APIs)
* android-tv/x86 (for Android TV)

Since the Target ABI is appended to the emulator file name, the plugin now tries to create files with forward slashes...
`Error: AVD name 'hudson_fr-FR_480_1080x1920_Google_Inc._Google_APIs_21_google-apis/armeabi-v7a' contains invalid characters.`

My commit replaces all the non alpha-numerial characters of the targetAbi to dashes.

If you are able, please merge this fix quickly, and deploy a new version of the plugin, as it impacts all the Android developers who want to run their tests on the latest versions of the OS :(

Merry christmas and happy new year !

Louis