(lang/R-cpp) Updated 0.2.4 to 0.2.7

# cpp11 0.2.7

* Fix a transient memory leak for functions that return values from
  `cpp11::unwind_protect()` and `cpp11::safe` (#154)

# cpp11 0.2.6

* `cpp_register()` now uses symbols exclusively in the `.Call()`
  interface. This allows it to be more robust in interactive use with
  the pkgload package.

# cpp11 0.2.5

* `cpp_source()` gains a `cxx_std` argument to control which C++
  standard is used.  This allows you to use code from `C++14` and
  later standards with cpp_source(). (#100)

* The cpp11 knitr engine now allows you to set the `cxx_std` chunk
  option to control the C++ standard used.

* `cpp_source()` now has much more informative error messages when
  compilation fails (#125, #139)

* `cpp_source()` now uses a unique name for the DLL, so works when run
  multiple times on the same source file on Windows (#143)

* `writable::list_of<T>` now supports modification of vectors as
  intended (#131).

* Errors when running
  `tools::package_native_routine_registration_skeleton()` are no
  longer swallowed (#134)

* `cpp_source()` can now accept a source file called `cpp11.cpp`
  (#133)

* `named_arg` now explicitly protect their values, avoiding protection
  issues when using large
  inputs. [tidyverse/readr#1145](https://github.com/tidyverse/readr/issues/1145)

* `r_string(std::string)` now uses `Rf_mkCharLenCE()` instead of
  `Rf_mkChar()`, which avoids the performance cost of checking the
  string length.

* Writable vector classes now properly set their lengths as intended
  when being copied to a read only class (#128).

(mef)