Try to fix stackless setup in travis config.

.travis.yml

@@ -91,7 +91,7 @@ branches:
 
 before_install:
   - |
-    if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then
+    if [ "$TRAVIS_OS_NAME" == "linux" ]; then
       # adding apt repos in travis is really fragile => retry a couple of times.
       for i in {1..10}; do travis_retry sudo apt-add-repository --yes 'ppa:ubuntu-toolchain-r/test' && break; sleep 2; done
       for i in {1..10}; do travis_retry sudo apt-get update && travis_retry sudo apt-get install --yes gcc-8  $(if [ -z "${BACKEND##*cpp*}" ]; then echo -n "g++-8"; fi ) && break; sleep 2; done
@@ -102,12 +102,12 @@ before_install:
     fi
 
   - |
-    if [[ "$TRAVIS_OS_NAME" == "osx" ]] || [[ "$STACKLESS" == "true" ]]; then # Install Miniconda
-      if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then CONDA_PLATFORM=MacOSX; else CONDA_PLATFORM=Linux; fi
+    if [ "$TRAVIS_OS_NAME" == "osx" -o "$STACKLESS" == "true" ]; then # Install Miniconda
+      if [ "$TRAVIS_OS_NAME" == "osx" ]; then CONDA_PLATFORM=MacOSX; else CONDA_PLATFORM=Linux; fi
       travis_retry curl -s -o miniconda.sh https://repo.continuum.io/miniconda/Miniconda$PY-latest-${CONDA_PLATFORM}-x86_64.sh
       bash miniconda.sh -b -p $HOME/miniconda && rm miniconda.sh
       #conda install --quiet --yes nomkl --file=test-requirements.txt --file=test-requirements-cpython.txt
-      if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then
+      if [ "$TRAVIS_OS_NAME" == "osx" ]; then
         which clang && clang --version && export CC=clang || true
         which clang++ && clang++ --version && export CXX=clang++ || true
       fi