FileContents.getLines performance fix

Inspired by #136 and #346 part. I was testing on the spring-boot project with the following config:

````
<module name="TreeWalker">
	<module name="LeftCurly"/>
	<module name="FallThrough"/>
	<module name="CustomImportOrder"/>
	<module name="LineLength"/>
	<module name="GenericWhitespace"/>
	<module name="NoWhitespaceAfter"/>
	<module name="NoWhitespaceBefore"/>
	<module name="OperatorWrap"/>
	<module name="WhitespaceAfter"/>
	<module name="WhitespaceAround"/>
</module>
````

Using checkstyle before this fix gived me 71 MB of String[] allocations. After this fix Only 2.4 MB of String[] were allocated. Execution time (that is not stable and was not properly measured) reduced insignificantly - both executions were less than 5s.