Reeplace deprecated function call.

Author: jgfoster

Diff for: lib/arduino_ci/cpp_library.rb

@@ -493,7 +493,7 @@ def build_for_test_with_configuration(test_file, aux_libraries, gcc_binary, ci_g
       base = test_file.basename
       # hide build artifacts
       build_dir = '.arduino_ci'
-      Dir.mkdir build_dir unless File.exists?(build_dir)
+      Dir.mkdir build_dir unless File.exist?(build_dir)
       executable = Pathname.new("#{build_dir}/unittest_#{base}.bin").expand_path
       File.delete(executable) if File.exist?(executable)
       arg_sets = []