Allow for warningless OTP 21 compilation and dialyzer'ing

[paulo-ferraz-oliveira]

We also make for an explicit minimum OTP 18 version (in rebar.config), as per .travis.yml's example.

[coveralls]

Coverage increased (+0.03%) to 75.405% when pulling 264c076 on paulo-ferraz-oliveira:feature/allow_otp_21 into 8ece9ec on elli-lib:develop.

[yurrriq]

LGTM, thanks!

[yurrriq]

@paulo-ferraz-oliveira, does kerl support OTP 21 yet? If so, would you mind adding it to the Travis build matrix?

[fenollp]

Why yes kerl works fine on debians, alpine & osx with 21.

[paulo-ferraz-oliveira]

@yurrriq: I'll add OTP 21 to the travis build matrix, since kerl supports it. Thanks for merging.

[paulo-ferraz-oliveira]

Now that I look at the PR, I don't think stacktrace_compat is required. Thing is I started the branch from master and later rebased on top of develop, but stacktrace_compat was already added. Let me check if it's possible to not have it (one less dep, :D)

[yurrriq]

Feel free to open another PR then. I'll try to be faster.

[paulo-ferraz-oliveira]

@yurrriq: I don't think it's necessary, as I explained earlier. If it becomes so, in the future, I'll open a new PR, sure.

[paulo-ferraz-oliveira]

Hi. Any chance this gets tagged soon?

[yurrriq]

Thanks for your patience! https://hex.pm/packages/elli/3.2.0

[paulo-ferraz-oliveira]

Sure, @yurrriq. Thank you.