Automatically install solc via solcx

[karlb]

Developers won't have to manually install solc, anymore.

[codecov]

Codecov Report

Merging #1452 (b7585ef) into master (e7dec86) will increase coverage by 0.03%.
The diff coverage is 100.00%.

❗ Current head b7585ef differs from pull request most recent head 207b8e2. Consider uploading reports for the commit 207b8e2 to get more accurate results

@@            Coverage Diff             @@
##           master    #1452      +/-   ##
==========================================
+ Coverage   80.61%   80.65%   +0.03%     
==========================================
  Files          22       22              
  Lines        1558     1561       +3     
  Branches      190      190              
==========================================
+ Hits         1256     1259       +3     
  Misses        255      255              
  Partials       47       47              

Impacted Files	Coverage Δ
raiden_contracts/contract_source_manager.py	94.38% <100.00%> (+0.19%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e7dec86...207b8e2. Read the comment docs.

[karlb]

@palango Do you have an idea why the second commit makes the solium installation fail?

I could just remove 207b8e2, but I would like to understand this first. One explanation would be that solium relies on an installed solc, but I saw this neither in the error message nor in the solium docs.

[palango]

@palango Do you have an idea why the second commit makes the solium installation fail?

No, doesn't make much sense from my pov either.

[karlb]

PR with only the first commit is #1453