Stop relying on `make benchmark`'s `-I$(srcdir)/benchmark/lib`

These days I don't use `make benchmark`. The YAML files should be
executable with bare `benchmark-driver` CLI without passing
`RUBYOPT=-Ibenchmark/lib`.

benchmark/mjit_exec_jt2jt.yml

@@ -1,6 +1,6 @@
 # Usage:
 #   RUBYOPT=-Ibenchmark/lib benchmark-driver -e 'ruby --jit' benchmark/mjit_exec_vm2jt.yml
-type: mjit_exec # benchmark/lib/benchmark_driver/runner/mjit_exec.rb
+type: lib/benchmark_driver/runner/mjit_exec
 num_methods: [1]
 #num_methods: (1..100).to_a + [200, 300, 400, 500, 600, 700, 800, 900, 1000]
 loop_count: 50000000

benchmark/mjit_exec_vm2jt.yml

@@ -1,6 +1,6 @@
 # Usage:
 #   RUBYOPT=-Ibenchmark/lib benchmark-driver -e 'ruby --jit' benchmark/mjit_exec_vm2jt.yml
-type: mjit_exec # benchmark/lib/benchmark_driver/runner/mjit_exec.rb
+type: lib/benchmark_driver/runner/mjit_exec
 num_methods: [1]
 #num_methods: (1..100).to_a + [200, 300, 400, 500, 600, 700, 800, 900, 1000]
 loop_count: 50000000

benchmark/mjit_exec_vm2vm.yml

@@ -1,6 +1,6 @@
 # Usage:
 #   RUBYOPT=-Ibenchmark/lib benchmark-driver -e 'ruby --jit' benchmark/mjit_exec_vm2vm.yml
-type: mjit_exec # benchmark/lib/benchmark_driver/runner/mjit_exec.rb
+type: lib/benchmark_driver/runner/mjit_exec
 num_methods: [1]
 #num_methods: (1..100).to_a + [200, 300, 400, 500, 600, 700, 800, 900, 1000]
 loop_count: 50000000

common.mk

@@ -47,7 +47,7 @@ GEM_PATH =
 GEM_VENDOR =
 
 BENCHMARK_DRIVER_GIT_URL = https://github.com/benchmark-driver/benchmark-driver
-BENCHMARK_DRIVER_GIT_REF = v0.15.13
+BENCHMARK_DRIVER_GIT_REF = v0.15.15
 SIMPLECOV_GIT_URL = https://github.com/colszowka/simplecov.git
 SIMPLECOV_GIT_REF = v0.17.0
 SIMPLECOV_HTML_GIT_URL = https://github.com/colszowka/simplecov-html.git