Enhancement: Remove solc-js compiler from the bundle

[CruzMolina]

This PR seeks to resolve #2034 by removing the solc-js compiler component completely from the bundle.

This is handled by:

• leveraging truffle obtain in a postinstall to download and cache a default soljson when installing truffle via npm.
• Importing only the solc/wrapper to parse soljson's.
• Removing the Bundled strategy from the CompilerSupplier.

The current implementation of this can be tested at truffle@unbundled-solc.

[gnidan]

Couple other things:

• Even though the compiler supplier default has changed, I think it's worth adding a separate truffle obtain step to the .travis.yml file, just so it doesn't make any single test slower than it should be.
• Probably worth including the README change so users know to <truffle?> obtain after yarn bootstrap we can do this later, if anyone complains
• Should we add the compilers directory to the Travis cache?

All in all, this is exciting stuff! I think we should wait for solc to at least respond before we start using that truffle-solc-wrapper, however.

[coveralls]

Coverage decreased (-0.4%) to 69.692% when pulling d5b6543 on wrapper-solc into 017a712 on develop.

[gnidan]

🎈 woot!