Import reference fails under php 7

[mishal]

Import by reference test fails under php 7.

The test file: less.js/less/import-reference.less

There was 1 error:

1) ILess\Test\CachingTest::testCompilation with data set #24 ('/home/travis/build/mishal/ile...e.less', '/home/travis/build/mishal/ile...ce.css')

unserialize(): Error at offset 390 of 618 bytes
/home/travis/build/mishal/iless/lib/ILess/Node/ImportNode.php:333
/home/travis/build/mishal/iless/lib/ILess/Node/RulesetNode.php:1013
/home/travis/build/mishal/iless/lib/ILess/ImportedFile.php:159
/home/travis/build/mishal/iless/lib/ILess/FileInfo.php:93
/home/travis/build/mishal/iless/lib/ILess/Cache/Cache.php:68
/home/travis/build/mishal/iless/lib/ILess/Cache/FileSystemCache.php:150
/home/travis/build/mishal/iless/lib/ILess/Parser.php:118
/home/travis/build/mishal/iless/lib/ILess/Parser/Core.php:409
/home/travis/build/mishal/iless/tests/ILess/Test/Parser/ParsingTest.php:98

see: https://travis-ci.org/mishal/iless/jobs/86137704

separated from #62