Force the build of libcurl instead of using system one.

On ubuntu, the libcurl package is compiled with libkrb (kerberos).
However, no libkrb5.a is provided and so, we cannot link statically.

As we are not using kerberos, we can (and must) compile ourselves libcurl.

travis/Dockerfile

@@ -33,7 +33,6 @@ RUN \
     ctpp2-utils \
     libctpp2-dev \
     libmicrohttpd-dev \
-    libcurl4-openssl-dev \
 # Qt packages
     libqt5gui5 \
     qtbase5-dev \