Ensure CI always validate builtin test cases

Install necessary Python packages to satisfy the dependency of builtin
test cases including pip3. clang package was removed to eliminate
complexity with pip though.

TODO: it takes much longer time to set up Python packages rather than
compiling and validating dcurl itself.

.travis.yml

@@ -1,8 +1,14 @@
 language: c
 
 compiler:
-  - clang
   - gcc
 
+before_install:
+  - sudo apt-get install -y build-essential
+  - sudo apt-get install -y python3-pip
+  - sudo apt-get install -y python3-cffi
+  - sudo pip3 install --upgrade pip setuptools
+  - sudo pip3 install --ignore-installed pyota
+
 script:
-  - make
+  - make check