Messages related to NOPBC keyword were wrong

Thanks to @valsson

Includes mention to ABIN code

See https://groups.google.com/d/msgid/plumed-users/8fcca53b-38d1-470e-a413-a4339702c6db%40googlegroups.com

Thanks to Qingh Liao

osx8.2 is deprecated.

Problem was likely due to an upgrade of mpi in homebrew on travis

There was some "return 1". Now that exit code is checked, this would
have triggered an error

Report a failure in the following two cases:
- unknown test type
- test program exiting with non-zero code

While doing plumedcheck, verify that autoconf has been applied correctly

pushdoc script uses it to know if branches are supported or not

Merging debug flags and doc build (949397c0)
was a bad idea since the resulting job takes too long (doesn't
fit in 50 minutes for master branch)

I split them again, resulting in 6 jobs.

goes with a5daa52d

changed in order to work according to
e9aa6730

appliable -> applicable

failures are moved to the end of the list

(suggested by @valsson)

manual is always built. Developer doc has been updated accordingly
A total of 5 travis jobs is run each time

See #286

[makedoc]

Added in the list of options to be tested:
$($CXX --print-prog-name=ld) -r -o
As suggested by @alalazo

Since this option appears to work also with old compilers, I try
it as a first option.

See #291