add PHP version 5.5 to travis config
add zend config as requirement
use custom github repository for travis ci
add additional decencies in cause of zf2 skeleton app dependency
code style fixes for passing php-cs-fixer rules
add build status image of forked repository
add concat resolver for files with factory

RWOverdijk/AssetManager#110
add tests for ConcatResolver

RWOverdijk/AssetManager#110
set constructor variable for content optional
append content method to concat string asset
add php doc
make concat resolver mime resolver aware
fetch mime type and throw exception if mime type changes in concatenated files
removed unused code
create concat string asset instance  and add content in loop
removed appending content to variable in loop
apply filters to assets
set mimetype in loop
set target path of resolved concat string asset
update tests