Fix pre-build issues [Travis CI]

[areebbeigh]

I'm submitting a ...

• [x] bug report
• [ ] feature request

Currently the Travis builds are failing before running the tests at the following points during linting:

• /app/scrapers/yahoo.py:82:17

  $ flake8 . --count --max-complexity=16 --show-source --statistics
  ./app/server.py:12:1: F841 local variable 'e' is assigned to but never used
  except Exception as e:
  ^
  ./app/scrapers/yahoo.py:82:17: W605 invalid escape sequence '\d'
          cleanl = re.compile('&#[\d]+(;)')
              ^
  1     F841 local variable 'e' is assigned to but never used
  1     W605 invalid escape sequence '\d'

• ./app/server.py:12:1

  $ flake8 . --count --max-complexity=16 --show-source --statistics
  ./app/server.py:12:1: F841 local variable 'e' is assigned to but never used
  except Exception as e:
  ^
  1     F841 local variable 'e' is assigned to but never used

Before running tests:

• There's also a dependency clash in dev packages preventing pytest-cov>=2.6.0 from being loaded.

I've already fixed these pre-build issues and Travis is successfully moving on to running the actual tests (which are failing due to other reasons).

[areebbeigh]

I'll create a PR after confirmation from maintainers.