Improve robustness of MavenProjectXmlWriter

Using the [bootstrap plugin](http://gradle.org/docs/current/userguide/bootstrap_plugin.html), I kept getting this error:

`[Fatal Error] :346:6: The processing instruction target matching "[xX][mM][lL]" is not allowed.
:maven2Gradle FAILED`

Turns out the reason was that my pom.xml had this at the top:
`<?xml version="1.0" encoding="UTF-8"?>`

instead of what the plugin was expecting. 
`<?xml version="1.0"?>`

This changes makes the search and replace more robust using a regex expression to 
identify to the XML declaration.