travis: Use $(...) instead ... in bash scripts

It's encouraged to use version with $(...).
    Backtick command substitution `...` is legacy
    syntax with several issues.

   1. It has a series of undefined behaviors related to quoting in POSIX.
   2. It imposes a custom escaping mode with surprising results.
   3. It's exceptionally hard to nest.

   ~https://github.com/koalaman/shellcheck/wiki/SC2006

Signed-off-by: Karol Trzcinski <[email protected]>

.travis.yml

@@ -12,4 +12,4 @@ jobs:
       before_install: docker pull thesofproject/sof && docker tag thesofproject/sof sof
       script:
         - echo -e '#!/bin/bash\nmkdir build && cd build && cmake .. && make -Werror -Wall -Wmissing-prototypes -Wimplicit-fallthrough=3 -Wpointer-arith' > build.sh && chmod +x build.sh
-        - docker run -i -t -v `pwd`:/home/sof/work/sof.git --user `id -u` sof ./build.sh
+        - docker run -i -t -v $(pwd):/home/sof/work/sof.git --user $(id -u) sof ./build.sh