Use libcurl4-gnutls-dev in test-external
Use libcurl4-gnutls-dev in test-external

test-external are failing with 302 errors.
Attempts to fix them need to work on Travis CI.

Noticed that Travis these days has the aforementioned library
installed by default, and it removes it to install the one I had
specified before.  Still leaving it explicitly mentioned.

parapluu/Concuerror,parapluu/Concuerror,aronisstav/Concuerror,aronisstav/Concuerror