Install nvm-exec as well when installing via script.

Fixes #553.
diff --git a/install.sh b/install.sh
index 5296e47..ae9d866 100755
--- a/install.sh
+++ b/install.sh
@@ -28,6 +28,8 @@
   fi
   if [ "_$NVM_METHOD" = "_script" ]; then
     NVM_SOURCE="https://raw.githubusercontent.com/creationix/nvm/v0.18.0/nvm.sh"
+  elif [ "_$NVM_METHOD" = "_script-nvm-exec" ]; then
+    NVM_SOURCE="https://raw.githubusercontent.com/creationix/nvm/v0.18.0/nvm-exec"
   elif [ "_$NVM_METHOD" = "_git" ] || [ -z "$NVM_METHOD" ]; then
     NVM_SOURCE="https://github.com/creationix/nvm.git"
   else
@@ -74,6 +76,8 @@
 install_nvm_as_script() {
   local NVM_SOURCE
   NVM_SOURCE=$(nvm_source "script")
+  local NVM_EXEC_SOURCE
+  NVM_EXEC_SOURCE=$(nvm_source "script-nvm-exec")
 
   # Downloading to $NVM_DIR
   mkdir -p "$NVM_DIR"
@@ -83,9 +87,17 @@
     echo "=> Downloading nvm as script to '$NVM_DIR'"
   fi
   nvm_download -s "$NVM_SOURCE" -o "$NVM_DIR/nvm.sh" || {
-    echo >&2 "Failed to download '$NVM_SOURCE'.."
+    echo >&2 "Failed to download '$NVM_SOURCE'"
     return 1
   }
+  nvm_download -s "$NVM_EXEC_SOURCE" -o "$NVM_DIR/nvm-exec" || {
+    echo >&2 "Failed to download '$NVM_EXEC_SOURCE'"
+    return 2
+  }
+  chmod a+x "$NVM_DIR/nvm-exec" || {
+    echo >&2 "Failed to mark '$NVM_DIR/nvm-exec' as executable"
+    return 3
+  }
 }
 
 #
diff --git a/test/install_script/nvm_source b/test/install_script/nvm_source
index 94c773f..8307639 100755
--- a/test/install_script/nvm_source
+++ b/test/install_script/nvm_source
@@ -17,6 +17,9 @@
 # nvm_source with script parameter returns the location of nvm.sh
 echo $(nvm_source "script") | grep "nvm.sh$" > /dev/null || die "nvm_source \"script\" should return the location of nvm.sh"
 
+# nvm_source with script-nvm-exec parameter returns the location of nvm-exec
+echo $(nvm_source "script-nvm-exec") | grep "nvm-exec$" > /dev/null || die "nvm_source \"script-nvm-exec\" should return the location of nvm.sh"
+
 # nvm_source with any other parameter errors out and exits
 nvm_source "anything" 2> /dev/null && die "nvm_source with invalid parameter should exit"
 out=$(nvm_source "anything" 2>&1 >/dev/null) || : #Saving the process here