CI should build a perl with EUMM master as its EUMM

It would take latest stable release perl, and overlay git master EUMM with something like, in perl src dir:

``` sh
# git clean -xdf
cp -r ../ExtUtils-MakeMaker/lib/ExtUtils/* cpan/ExtUtils-MakeMaker/lib/ExtUtils
( ./Configure -des -Dusedevel && make -j4 test 2>&1) | tee ../pbt-log
```
