Intermittent CI failures with PDF builds

[mortenpi]

The "PDF/LaTeX backend" stage often fails on CI, causing the docs not to deploy (since they only deploy if that stage passes):

PDF/LaTeX: simple: Test Failed at /home/travis/build/JuliaDocs/Documenter.jl/test/examples/tests_latex.jl:24
  Expression: joinpath(build_dir, "DocumenterLaTeXSimple.pdf") |> isfile
Stacktrace:
 [1] macro expansion at /home/travis/build/JuliaDocs/Documenter.jl/test/examples/tests_latex.jl:24 [inlined]
 [2] macro expansion at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.0/Test/src/Test.jl:1083 [inlined]
 [3] macro expansion at /home/travis/build/JuliaDocs/Documenter.jl/test/examples/tests_latex.jl:21 [inlined]
 [4] macro expansion at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.0/Test/src/Test.jl:1083 [inlined]
 [5] top-level scope at /home/travis/build/JuliaDocs/Documenter.jl/test/examples/tests_latex.jl:20
PDF/LaTeX: Test Failed at /home/travis/build/JuliaDocs/Documenter.jl/test/examples/tests_latex.jl:32
  Expression: joinpath(build_dir, "DocumenterLaTeX.pdf") |> isfile
Stacktrace:
 [1] macro expansion at /home/travis/build/JuliaDocs/Documenter.jl/test/examples/tests_latex.jl:32 [inlined]
 [2] macro expansion at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.0/Test/src/Test.jl:1083 [inlined]
 [3] macro expansion at /home/travis/build/JuliaDocs/Documenter.jl/test/examples/tests_latex.jl:29 [inlined]
 [4] macro expansion at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.0/Test/src/Test.jl:1083 [inlined]
 [5] top-level scope at /home/travis/build/JuliaDocs/Documenter.jl/test/examples/tests_latex.jl:20
Test Summary:       | Pass  Fail  Total
Examples/LaTeX      |    2     2      4
  PDF/LaTeX: simple |    1     1      2
  PDF/LaTeX         |    1     1      2
ERROR: LoadError: Some tests did not pass: 2 passed, 2 failed, 0 errored, 0 broken.
in expression starting at /home/travis/build/JuliaDocs/Documenter.jl/test/examples/tests_latex.jl:19

Restarting the build usually fixes it though.