Prefer python3 over python(2) in checkout-deps (#1424)

* Prefer python3 over python(2) in checkout-deps

We require Python 3 and don't want to accidentally use Python 2 if both are available.

* Use python3 in travis build

* Install python3-pip package on travis trusty image

* Fix pip detection in checkout-deps.sh

tools/checkout-deps.ps1

@@ -92,10 +92,10 @@ $SDKS | ForEach-Object {
 }
 
 # Find a suitable installation of Python
-$PYTHON_CMD = Get-Command 'python' -ErrorAction SilentlyContinue
+$PYTHON_CMD = Get-Command 'python3' -ErrorAction SilentlyContinue
 if ($NULL -eq $PYTHON_CMD)
 {
-    $PYTHON_CMD = Get-Command 'python3' -ErrorAction SilentlyContinue
+    $PYTHON_CMD = Get-Command 'python' -ErrorAction SilentlyContinue
     if ($NULL -eq $PYTHON_CMD)
     {
         $PYTHON_CMD = Get-Command 'py' -ErrorAction SilentlyContinue

tools/checkout-deps.sh

@@ -143,9 +143,9 @@ do
   checkout
 done
 
-python_cmd=`command -v python`
+python_cmd=`command -v python3`
 if [ -z "$python_cmd" ]; then
-  python_cmd=`command -v python3`
+  python_cmd=`command -v python`
 
   if [ -z "$python_cmd" ]; then
     echo "No suitable installation of Python detected"
@@ -153,11 +153,11 @@ if [ -z "$python_cmd" ]; then
   fi
 fi
 
-`$python_cmd -c "import ambuild2"` 2>&1 1>/dev/null
+$python_cmd -c "import ambuild2" 2>&1 1>/dev/null
 if [ $? -eq 1 ]; then
   echo "AMBuild is required to build SourceMod"
 
-  `$python_cmd -m pip --version` 2>&1 1>/dev/null
+  $python_cmd -m pip --version 2>&1 1>/dev/null
   if [ $? -eq 1 ]; then
     echo "The detected Python installation does not have PIP"
     echo "Installing the latest version of PIP available (VIA \"get-pip.py\")"