Improve Travis test coverage and simplify config

Specify the xenial (16.04) image, as Travis still sometimes uses 14.04
by default.  Use cmocka from the package repository instead of
building it.  Remove python3-paste as paste is no longer a test
dependency, but add python3-kdcproxy.  Install pyrad via pip3 as the
Ubuntu-packaged version still has the assertion bug.

.travis.yml

@@ -2,6 +2,8 @@ language: c++
 
 sudo: required
 
+dist: xenial
+
 matrix:
   include:
   - compiler: clang
@@ -14,15 +16,7 @@ matrix:
 
 before_install:
   - sudo apt-get update -qq
-  - sudo apt-get install -y bison dejagnu gettext keyutils ldap-utils libldap2-dev libkeyutils-dev libssl-dev python3-paste slapd tcl-dev tcsh
-  - mkdir -p cmocka/build
-  - cd cmocka
-  - wget https://cmocka.org/files/1.1/cmocka-1.1.1.tar.xz
-  - tar -xvf cmocka-1.1.1.tar.xz
-  - cd build
-  - cmake ../cmocka-1.1.1 -DCMAKE_INSTALL_PREFIX=/usr
-  - make
-  - sudo make install
-  - cd ../..
+  - sudo apt-get install -y bison dejagnu gettext keyutils ldap-utils libcmocka-dev libldap2-dev libkeyutils-dev libssl-dev python3-kdcproxy python3-pip slapd tcl-dev tcsh
+  - pip3 install pyrad
 
 script: sh -ex .travis-ci.sh