fixed java.io.File handling in less compiler

Problem:
Changes to lib.less will be ignored by play in dev mode.
```css
# File "test.less"
@import "lib.less"

#File "lib.less"
body {
    color: red;
}
````

The fix:
The lesscompiler returns relative pathes. My fixed converts relative paths to absolutes paths. The dependencies variable in the AssetCompiler does not work with mixed relative/absolute pathes.
scala shell example (path "/var/share/man" choosen by random):
```text
scala> val absolute = new java.io.File("/var/share/man")
absolute: java.io.File = /var/share/man
scala> absolute.equals(absolute)
res0: Boolean = true
scala> val relative = new java.io.File("/var/share/doc/../man")
relative: java.io.File = /var/share/doc/../man
scala> absolute.equals(relative)
res1: Boolean = false
scala> absolute.equals(relative.getCanonicalFile)
res2: Boolean = true
```