Fix bug #703: update version number for shared library

When update next version number, look at differences for e.g. b47a75f4 commit
and update following files:
  - CHANGELOG.md
  - android/jni/include/config.h
  - configure.ac
  - src/windows/config.h
  - Makefile.am

Makefile.am

@@ -11,7 +11,7 @@ VIMDIR=$(PREFIX)/share/vim
 pkgconfigdir = $(libdir)/pkgconfig
 pkgconfig_DATA = espeak-ng.pc
 
-SHARED_VERSION=2:49:1 # = *.so.1.1.49
+SHARED_VERSION=2:51:1 # = *.so.1.1.51
 
 MKDIR=mkdir -p