Install pip if it's not available at all

This is not supposed to happen, but somehow it does on Travis CI, so try
to remedy this by installing pip via apt if it's missing.

build/tools/httpbin.sh

@@ -37,6 +37,11 @@ httpbin_launch() {
             # Elsewhere just use Python 3.
             PY3=3
 
+            # Ensure that we have at least some version of pip.
+            if ! python3 -m pip; then
+                sudo apt-get -q -o=Dpkg::Use-Pty=0 install python3-pip
+            fi
+
             # Running pip install fails with weird errors out of the box when
             # using old pip version because it attempts to use python rather
             # than python3, so upgrade it to fix this.