Electron 3,4,5 pre builts by add libpng function prefixes

[julianhille]

This prefixing added in version 1.6 of libpng does solve the version conflict of electrons libpng 1.2 and pdfwriter 1.6 by prefixing the exported functions and used functions of pdfwriter.

This also adds some more minor electron version together with xvfb fixes and streamlining of electron tests.

Electron 2.0.x might have never worked for dfonts cause it was tested against the node / nvm version instead of electron.

I created https://github.com/galkahana/HummusJS/issues/411 and excluded 2.0.x from pre built binaries as they do not pass tests.

[galkahana]

some mixup in my travis setup due to org->com change. i'll review and merge...just want to run some tests.

[julianhille]

Great thanks!