Impoved docs build

[Phally]

I've improved the docs build a bit, so that it's up-to-date and easier to contribute to it:

1. Old Docker config and docs gave some issues, so set Docker to the latest Debian release.

2. Added docs build and checks to Travis CI.

3. Upgraded the docs build to Python 3.

4. Added reStructuredText lint using RSTcheck.

5. Fixed an issue RSTcheck flagged. See first paragraph: https://crud.readthedocs.io/en/latest/unit-testing.html

6. Added Browser Sync config for easier editing.

   Does an automated build and an automated browser refresh when saving a file:

   Better resolution: https://youtu.be/NnocyVp-KHc

   

[codecov[bot]]

Codecov Report

Merging #640 into master will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #640   +/-   ##
=========================================
  Coverage     87.82%   87.82%           
  Complexity      407      407           
=========================================
  Files            36       36           
  Lines          1150     1150           
=========================================
  Hits           1010     1010           
  Misses          140      140           

---

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 a58c8b7...b5e0814. Read the comment docs.

[Phally]

Alright, this ready to be reviewed

[ADmad]

@josegonzalez Is this good to merge? :)

[jippi]

@Phally @ADmad looks good to me once merge conflict has been fixed :)

[ADmad]

@Phally Hi, would you mind re-opening this against the latest master?