Add automated CI tests for the GAP.prompt()

[fingolfin]

Right now I am not aware of a "good" way to test GAP.prompt(); but I'd really like to have one, so that when an issue like #458 pops up, then the fix (here in #459) can also include a test that verifies the issue is gone.

One way to go about this might be to use pexpect which can be used to automate interactive command line utilities. (Of course if somebody knows a pure-Julia way to do the same, I'd also be happy to use that). This can also be handy for other kinds of tests, as it allows to emulate what an actual user would experience very closely.

I think we need to do roughly this:

1. add pip install pexpect into .travis.yml
2. add a script to etc which performs the required test using pexpect
3. call that script from .travis.yml

[fingolfin]

Perhaps testing the Help system might also benefit from such a thing? @ThomasBreuer ?

[fingolfin]

Sp perhaps we should just use the Python package pexpect, it's not like it is hard; a simple attempt:

>>> import pexpect
>>> child = pexpect.spawn('julia-1.5 --project=proj-1.5')
>>> child.expect('julia> ')
0
>>> child.sendline('using GAP')
10
>>> child.expect('julia> ')
0
>>> child.sendline('GAP.prompt()')
13
>>> child.expect('gap> ')
0
>>> child.sendline('1+1;')
5
>>> child.expect('gap> ')
0
>>> child.sendline('1:1;')
5
>>> child.expect('gap> ')
0
>>> child.sendline('Error("foo");')
14
>>> child.expect('brk> ')
0
>>> child.sendline('quit;')
6
>>> child.sendline('quit;')
6
>>> child.expect('julia> ')
0

[ThomasBreuer]

Not only the help system, there are also other interactive features like Julia's REPL menus which can be tested then.

[fingolfin]

Just learned about https://gitlab.com/wavexx/Expect.jl which we can use for this, too