Ensure that a system has manpath before invoking it.

Closes #531, fixes #557.

nvm.sh

@@ -836,6 +836,7 @@ nvm() {
       PATH=`nvm_strip_path "$PATH" "/bin"`
       # Prepend current version
       PATH=`nvm_prepend_path "$PATH" "$NVM_VERSION_DIR/bin"`
+      if nvm_has manpath; then
         if [ -z "$MANPATH" ]; then
           MANPATH=$(manpath)
         fi
@@ -843,13 +844,14 @@ nvm() {
         MANPATH=`nvm_strip_path "$MANPATH" "/share/man"`
         # Prepend current version
         MANPATH=`nvm_prepend_path "$MANPATH" "$NVM_VERSION_DIR/share/man"`
+        export MANPATH
+      fi
       # Strip other version from NODE_PATH
       NODE_PATH=`nvm_strip_path "$NODE_PATH" "/lib/node_modules"`
       # Prepend current version
       NODE_PATH=`nvm_prepend_path "$NODE_PATH" "$NVM_VERSION_DIR/lib/node_modules"`
       export PATH
       hash -r
-      export MANPATH
       export NODE_PATH
       export NVM_PATH="$NVM_VERSION_DIR/lib/node"
       export NVM_BIN="$NVM_VERSION_DIR/bin"