README: require Boost 1.53 or newer instead of 1.56

[AdrianDC]

Works properly based on Boost 1.53, therefore no backports from Boost 1.56 are required.

The RPM and DEB cmake rules already specify the following requirements:

./cmake/CPackConfigRPM.cmake:    SET(CPACK_RPM_PACKAGE_REQUIRES "libboost >= 1.53")                                 
./cmake/CPackConfigDEB.cmake:    SET(CPACK_DEBIAN_PACKAGE_DEPENDS "libboost (>= 1.53)")                                    
./cmake/CPackConfigDEB.cmake:    SET(CPACK_DEBIAN_PACKAGE_BUILDS_DEPENDS "libboost-dev (>= 1.53)")                                                            

And the Travis configuration uses 1.55.

[Jeffail]

Awesome, thanks @AdrianDC.