travis: Install pyelftools via pip

With the migration to python3 for all of our tests, we need to install
pyelftools via pip now rather than the system tools as they will
otherwise not be present in our virtualenv.

Signed-off-by: Tom Rini <trini@konsulko.com>
---
Changes in v2: Switch to pip

.travis.yml

@@ -23,7 +23,6 @@ addons:
     - build-essential
     - libsdl2-dev
     - python
-    - python-pyelftools
     - python3-sphinx
     - python3-virtualenv
     - python3-pip
@@ -256,6 +255,7 @@ script:
      if [[ -n "${TEST_PY_TOOLS}" ]]; then
        export PYTHONPATH="${UBOOT_TRAVIS_BUILD_DIR}/scripts/dtc/pylibfdt";
        export PATH="${UBOOT_TRAVIS_BUILD_DIR}/scripts/dtc:${PATH}";
+       pip install pyelftools &&
        ./tools/binman/binman --toolpath ${UBOOT_TRAVIS_BUILD_DIR}/tools test &&
        ./tools/patman/patman test &&
        ./tools/buildman/buildman -t &&