Fixed CONDA with MacOS on Travis

.travis.yml

@@ -95,7 +95,7 @@ matrix:
   - name: Conda build, MacOS
     os: osx
     if: branch =~ ^test- OR tag IS present OR type IN(pull_request) OR type IN(cron)
-    osx_image: xcode8.3
+    osx_image: xcode9.2
     env: PLUMED_CONDA=yes
 # I don't put any mpi variant with macports since it takes too long to compile
 ## This command can be used to allow failures:

conda/plumed/conda_build_config.yaml

+CONDA_BUILD_SYSROOT:
+  - /var/tmp/MacOSX-SDKs/MacOSX10.9.sdk   # [osx]

conda/py-plumed/conda_build_config.yaml

+CONDA_BUILD_SYSROOT:
+  - /var/tmp/MacOSX-SDKs/MacOSX10.9.sdk   # [osx]
 python:
     - 2.7
     - 3.6

conda/run-conda-build.sh

@@ -9,6 +9,14 @@ if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then
     csys=Linux
 elif [[ "$TRAVIS_OS_NAME" == "osx" ]]; then
     csys=MacOSX
+    PREV=$(pwd)
+    cd /var/tmp
+    mkdir MacOSX-SDKs
+    cd MacOSX-SDKs
+    wget https://github.com/phracker/MacOSX-SDKs/releases/download/10.13/MacOSX10.9.sdk.tar.xz
+    tar -xf ./MacOSX10.9.sdk.tar.xz
+    rm MacOSX10.9.sdk.tar.xz
+    cd $PREV
 else
     echo "Unsupported system $TRAVIS_OS_NAME"
     exit 1