chore(travis): Combine code coverage build with full build

Saves time on the CI job, and the produced binary isn't used for releases
so it's uneeded to keep it unmodified.

.travis.yml

@@ -47,12 +47,12 @@ jobs:
       env: JOB=build-ubuntu-16-04
       addons:
         apt:
-          packages: 
+          packages:
             - lcov
       script: "./.travis/$JOB.sh"
       after_success:
         # Create lcov report
-        - lcov --directory _debug --capture --output-file coverage.info
+        - lcov --directory _build --capture --output-file coverage.info
         # Filter out system headers and test sources
         - lcov --remove coverage.info '/usr/*' '*/test/*' '*/*_autogen/*' --output-file coverage.info
         # Upload report to codecov.io 

.travis/build-ubuntu-16-04.sh

@@ -198,20 +198,15 @@ build_qtox() {
     echo '*** BUILDING "FULL" VERSION ***'
     cmake -H. -B"$BUILDDIR" \
         -DUPDATE_CHECK=ON \
-        -DSTRICT_OPTIONS=ON
+        -DSTRICT_OPTIONS=ON \
+        -DCODE_COVERAGE=ON
     bdir
 }
 
 test_qtox() {
-    local BUILDDIR=_debug
-
-    cmake -H. -B"$BUILDDIR" \
-        -DUPDATE_CHECK=ON \
-        -DSTRICT_OPTIONS=ON \
-        -DCODE_COVERAGE=ON
+    local BUILDDIR=_build
 
     cd $BUILDDIR
-    make -j$(nproc)
     make test
     cd -
 }