Add test coverage tool excoveralls to see where one might imporve

[PragTob]

Remembered saying that a couple of tests were still missing, so
this would be a nice first step to get them there.

If you create an account at coveralls
and activate it then mix coveralls.travis could be used as
the test script to collect test coverage data on CI run
(I have this setup for benchee)

That also comes with a nice little badge :)

FYI as of this commit:

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COV FILE LINES RELEVANT MISSED
100.0% lib/wallaby.ex 59 10 0
74.6% lib/wallaby/browser.ex 1091 240 61
0.0% lib/wallaby/dsl.ex 12 0 0
76.9% lib/wallaby/element.ex 184 26 6
64.3% lib/wallaby/exceptions.ex 93 14 5
100.0% lib/wallaby/helpers/key_codes.ex 74 6 0
8.3% lib/wallaby/metadata.ex 51 12 11
73.3% lib/wallaby/phantom.ex 83 15 4
67.8% lib/wallaby/phantom/driver.ex 428 118 38
100.0% lib/wallaby/phantom/log_store.ex 26 7 0
100.0% lib/wallaby/phantom/logger.ex 23 6 0
96.7% lib/wallaby/phantom/server.ex 105 30 1
100.0% lib/wallaby/query.ex 289 35 0
78.6% lib/wallaby/query/error_message.ex 173 28 6
100.0% lib/wallaby/query/xpath.ex 77 11 0
0.0% lib/wallaby/session.ex 58 2 2
100.0% test/support/session_case.ex 19 4 0
57.1% test/support/test_server.ex 27 7 3
[TOTAL] 76.0%

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Generating report...

I also promise, this is the last PR for today 😁

[keathley]

Send as many PRs as you want! I think adding coveralls is fine, especially if it can help folks get involved. I added wallaby to coveralls so we should be able to update the travis config and start capturing those data.