Failing doctests should fail the build

[schillic]

@mforets: Currently there is a failing doctest in master:

> File: .../LazySets/src/Ball1.jl

> Code block:                                                                                                       

\`\`\`jldoctest
julia> σ([0.,1], B)
1-element Array{Float64,1}:
 1.0
 0.0
\`\`\`

> Subexpression:                                                                                                    

    σ([0.,1], B)

> Output Diff:                                                                                                      

1-element 2-element Array{Float64,1}:
 1.0
 0.00.0
 1.0 

However, Travis does not complain.
See here how to potentially implement this.

[schillic]

In my recent commits I did not include unit tests but only doctests. Assuming this issue is resolved, they should be equivalent from the automatic builds view (fails iff one of the tests breaks).

+ Get doc examples for free.
- Usually you only run the unit tests locally; then you do not see if you broke something.

I do not have a strong opinion here. We could also have both, but then one of the tests is obviously redundant.

[schillic]

@mforets asked about this in the Documenter repo.

[mforets]

In #677 i set the strict flag build to false by default. The doctests were passing locally but there still the docs was not working.

I'm not proposing this measure to be definitive, but i'm reopening this issue to keep it in mind. Maybe we should add the strict option to be read in the command line, like

$ julia --color=yes docs/make.jl --strict=true

[schillic]

There is now a new documentation for using build stages. After the jump to v1.0, we should try this again.

[schillic]

Now that we have #1351, I would say we can close this.