travis: Fix unquoted ${COVERITY_SCAN_BRANCH} test
travis: Fix unquoted ${COVERITY_SCAN_BRANCH} test

This fixes PR builds missing the actual `make` step due to a bash
syntax error in case when the ${COVERITY_SCAN_BRANCH} expands to
empty:

    $ if [ ${COVERITY_SCAN_BRANCH} != 1 ]; then make && ./scripts/continuous/run.sh "$TEMPLATE"; fi
    /home/travis/build.sh: line 45: [: !=: unary operator expected

    The command "if [ ${COVERITY_SCAN_BRANCH} != 1 ]; then make && ./scripts/continuous/run.sh "$TEMPLATE"; fi" exited with 0.

The worst thing is that this error used to make PR builds effectively
no-op, resulting in a green build without any check.

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