Allow for possible global commands

`python` and `rspec` might exist globally

test/exec.bats

@@ -16,10 +16,10 @@ create_executable() {
 
 @test "fails with invalid version" {
   export PYENV_VERSION="3.4"
-  run pyenv-exec python -V
+  run pyenv-exec nonexistent
   assert_failure <<EOF
 pyenv: version \`3.4' is not installed (set by PYENV_VERSION environment variable)
-pyenv: python: command not found
+pyenv: nonexistent: command not found
 EOF
 }
 
@@ -27,10 +27,10 @@ EOF
   mkdir -p "$PYENV_TEST_DIR"
   cd "$PYENV_TEST_DIR"
   echo 2.7 > .python-version
-  run pyenv-exec rspec
+  run pyenv-exec nonexistent
   assert_failure <<EOF
 pyenv: version \`2.7' is not installed (set by $PWD/.python-version)
-pyenv: rspec: command not found
+pyenv: nonexistent: command not found
 EOF
 }