add support for Java 1.8

[bitdream]

To be used with more platforms, it's important to remove the language level dependency and third party library dependency. For these reasons I have to use other over simplified GIF readers for now but hope in future I could try your library...
I do suggest:
removing the language level requirement, so support Java 1.8
removing all the third party library dependencies.

[ribasco]

removing the language level requirement, so support Java 1.8

There are language specific features in this project which requires at least versions 10 or above. That said, I have no plans of supporting java 8.

removing all the third party library dependencies.

The only library that is used is commons-compress (the rest are just used for development and testing). I have no problem removing this dependency as long as there is an alternative implementation for BitInputStream. If you're up to it, feel free to send a pull request and I'll gladly merge it.