Add a way to pass the `evmVersion` parameter
to `solc` by:

1. Simplify the `Compiler.compileJSON()` interface from
`compileJSON(source, optimize)` to `compileJSON(source)`

Whether optimization is enabled is then always determined by
the `Compiler.optimize` object field which simplifies data flow.

2. Add a `Compiler.evmVersion` field and a setter and
make `compileJSON()` honor that.

Specifying `evmVersion` is useful when targetting an older
private chain with the latest `solc` or when using
an EVM-compatible chain that's slightly behind Ethereum
main-net in terms of features.

The unused `userAgent` variable in `onInternalCompilerLoaded` was
only removed to fix `npm run test` for `remix-solidity`'.