fix: _compile_combined_json should use the given version of solc

[qnnnnez]

What I did

Make _compile_combined_json use the correct version of solc. Currently it will always use default version of solc.

How I did it

Make _compile_combined_json accept an argument solc_binary.

How to verify it

Test script:

import solcx

solcx.install_solc('latest')
solcx.install_solc('0.6.6')
solcx.compile_source('', solc_version='0.6.6')

Running the test script with current master:

solcx.exceptions.UnknownValue: solc 0.6.6 does not accept 'function-debug
' as an option for the '--combined-json' flag

Running the test script with the fix:

> command: `/Users/wushuang/.solcx/solc-v0.6.6 --combined-json abi,asm,ast,bin,bin-runtime,compact-format,devdoc,hashes,interface,metadata,opcodes,srcmap,srcmap-runtime,storage-layout,userdoc -`
> return code: `0`
> stdout:
{"sourceList":["<stdin>"],"sources":{"<stdin>":{"AST":{"absolutePath":"<stdin>","exportedSymbols":{},"id":1,"nodeType":"SourceUnit","nodes":[],"src":"1:0:0"}}},"version":"0.6.6+commit.6c089d02.Darwin.appleclang"}

> stderr:
Warning: Source file does not specify required compiler version! Consider adding "pragma solidity ^0.6.6;"
 --> <stdin>

Checklist

• [ ] I have confirmed that my PR passes all linting checks
• [ ] I have included test cases
• [ ] I have updated the documentation (README.md)
• [ ] I have added an entry to the changelog

[antazoey]

It looks like this got done somewhere else