Maintenance release

This release comes with a couple of improvements and bugfixes to make the library easier to use, there is no new big feature introduced.

Improvements:

• dependency version cleanup & removing guava - contributed by @snazarenko67
• EnumSchema now keeps the order of possible values as they were listed in the schema JSON - contributed by @cyqw
• schema loading: using the explicitly set spec version when loading schemas which have a "$schema" value but it doesn't point to any supported specification versions
• adding some type checking of subject under validation

Bugfixes:

• avoiding possible NPE in ValidationException#hashCode()
• properly handling Number subtypes in "number" keyword validation (AtomicLong, AtomicInteger, BigDecimal and BigInteger) - fixed by @EXPEaneparaczki
• typo fixes in the readme - contributed by @0xflotus