Bump version to VN:1.6 due to >65535 CIGAR strings

The new CG tag field BAM representation for long CIGAR strings
(PR samtools#227, merged as dab57f4)
will be unnoticed by older code. Such code will see the placeholder
CIGAR string, so it needs to be possible to signal the presence of
CG tags via the @HD-VN version number header field.

SAMv1.tex

@@ -59,7 +59,7 @@ \section{The SAM Format Specification}
 information such as mapping position, and variable number of optional
 fields for flexible or aligner specific information.
 
-This specification is for version 1.5 of the SAM and BAM formats. Each SAM and
+This specification is for version 1.6 of the SAM and BAM formats. Each SAM and
 BAM file may optionally specify the version being used via the
 {\tt @HD VN} tag. For full version history see Appendix~\ref{sec:history}. 
 
@@ -95,7 +95,7 @@ \subsection{An example}\label{sec:example}
 properly aligned\slash multiple segments.}
 \begin{framed}\small
 \begin{verbatim}
-@HD VN:1.5 SO:coordinate
+@HD VN:1.6 SO:coordinate
 @SQ SN:ref LN:45
 r001 99 ref 7 30 8M2I4M1D3M = 37 39 TTAGATAAAGGATACTG *
 r002 0 ref 9 30 3S6M1P1I4M * 0 0 AAAAGATAAGGATA *
@@ -707,7 +707,7 @@ \subsection{Padded SAM}\label{sec:padded-sam}
 
 \begin{framed}\small
 \begin{verbatim}
-@HD VN:1.5 SO:coordinate
+@HD VN:1.6 SO:coordinate
 @SQ SN:ref LN:47
 ref 516 ref 1 0 14M2D31M * 0 0 AGCATGTTAGATAAGATAGCTGTGCTAGTAGGCAGTCAGCGCCAT *
 r001 99 ref 7 30 14M1D3M = 39 41 TTAGATAAAGGATACTG *
@@ -1218,12 +1218,17 @@ \section{SAM Version History}\label{sec:history}
 specification with its own version numbering.\footnote{
 \href{http://samtools.github.io/hts-specs/SAMtags.pdf}{http://samtools.github.io/hts-specs/SAMtags.pdf}}
 
-\subsection*{1.5: 23 May 2013 to current}
+\subsection*{1.6: 28 November 2017 to current}
 
 \begin{itemize}
 \item Add {\tt @RG BC} header tag. (Apr 2018)
 \item Permit UTF-8 in a few header tags. (Mar 2018)
-\item Add support for $>65535$ cigar operations. (Jul 2017)
+\item\textbf{Add support for CIGAR strings with more than 65535 operations.} (Nov 2017)
+\end{itemize}
+
+\subsection*{1.5: 23 May 2013 to November 2017}
+
+\begin{itemize}
 \item Add {\tt @SQ AH} header tag. (Mar 2017)
 \item Auxiliary tags migrated to SAMtags document. (Sep 2016)
 \item Z and H auxiliary tags are permitted to be zero length. (Jun 2016)