Remove Boost deprecation warning

[mcol]

Summary

Replaces a deprecated header with the suggested one. Fixes #1672.

Tests

None.

Side Effects

None.

Checklist

• Math issue replace deprecated Boost header #1672

• Copyright holder: Marco Colombo

  The copyright holder is typically you or your assignee, such as a university or company. By submitting this pull request, the copyright holder is agreeing to the license the submitted work under the following licenses:
  - Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
  - Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

• the basic tests are passing

  • unit tests pass (to run, use: ./runTests.py test/unit)
  • header checks pass, (make test-headers)
  • dependencies checks pass, (make test-math-dependencies)
  • docs build, (make doxygen)
  • code passes the built in C++ standards checks (make cpplint)

[stan-buildbot]

---

Name	Old Result	New Result	Ratio	Performance change( 1 - new / old )
gp_pois_regr/gp_pois_regr.stan	4.94	4.91	1.01	0.57% faster
low_dim_corr_gauss/low_dim_corr_gauss.stan	0.02	0.02	0.97	-3.37% slower
eight_schools/eight_schools.stan	0.09	0.09	0.98	-2.52% slower
gp_regr/gp_regr.stan	0.22	0.22	0.99	-0.87% slower
irt_2pl/irt_2pl.stan	6.11	6.07	1.01	0.75% faster
performance.compilation	89.68	87.14	1.03	2.83% faster
low_dim_gauss_mix_collapse/low_dim_gauss_mix_collapse.stan	7.31	7.38	0.99	-0.9% slower
pkpd/one_comp_mm_elim_abs.stan	20.95	21.47	0.98	-2.51% slower
sir/sir.stan	90.49	93.84	0.96	-3.7% slower
gp_regr/gen_gp_data.stan	0.05	0.05	0.99	-0.72% slower
low_dim_gauss_mix/low_dim_gauss_mix.stan	3.0	2.99	1.0	0.23% faster
pkpd/sim_one_comp_mm_elim_abs.stan	0.31	0.31	0.99	-0.72% slower
arK/arK.stan	1.76	1.76	1.0	-0.22% slower
arma/arma.stan	0.81	0.81	1.0	-0.06% slower
garch/garch.stan	0.63	0.63	1.0	0.02% faster
Mean result: 0.992848921255

---

Jenkins Console Log
Blue Ocean
Commit hash: 56e93dd

---

Machine information

ProductName: Mac OS X ProductVersion: 10.11.6 BuildVersion: 15G22010

CPU:
Intel(R) Xeon(R) CPU E5-1680 v2 @ 3.00GHz

G++:
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 7.0.2 (clang-700.1.81)
Target: x86_64-apple-darwin15.6.0
Thread model: posix

Clang:
Apple LLVM version 7.0.2 (clang-700.1.81)
Target: x86_64-apple-darwin15.6.0
Thread model: posix

[rok-cesnovar]

LGTM!